The invasion of annual grasses Bromus tectorum and Ventenata dubia in the American west: competition and management

Abstract

Semi-arid rangelands in the American West have been invaded by non-native annual grasses, including Bromus tectorum and Ventenata dubia. These species threaten biodiversity, forage quality, and habitat. They have different spatial distributions but do sometimes co-occur, and concerns have been raised that V. dubia is replacing B. tectorum. Current and potentially greater range overlap should be considered in management contexts, as interactions between these species may enhance negative impacts or cause counter-effective responses to management. Management of invasive grasses is often achieved through herbicides, however, novel strategies like biofumigation should be studied. Biofumigation, the use of bio-active Brassica by-products, may target germinating seeds making it a potential tool for managing invasive annual grasses. The goal of this research was to elucidate the competitive dynamics between B. tectorum and V. dubia and determine whether biofumigation may be a management strategy. To quantify competitive interactions, an addition series study of B. tectorum and V. dubia was conducted. Both were negatively impacted by intraspecific and interspecific competition. To determine if plant-soil feedback (PSF) was affecting competition, I repeated the addition series in three soils (Neutral, B. tectorum, and V. dubia), over two experimental phases. Both were impacted by intraspecific competition in Phase 1, but by Phase 2 intraspecific competition was weakened in conspecific soil, suggesting a positive PSF. Next, I quantified the responses of B. tectorum, V. dubia, and two perennial grasses to biofumigation over two experiments. First, through a dose- response experiment with two biofumigants (seed meal and mulch). Seed meal reduced viability for our annual species and one perennial. Mulch was not consistent in reducing viability. The second study was performed in soil using multiple techniques (seed meal, seed meal leachate, straw, and straw leachate), and combined with solarization. Emergence was inhibited using seed meal treatments, but straw treatments and solarization were not effective. These studies have helped improve our knowledge of how these species interact and respond to biofumigation and ways we can use targeted management of these invasive grasses. Ultimately, the invasion of non- native annual grasses is a complex issue involving competitive dynamics and requiring multifaceted integrated weed management strategies.